Wood preservatives known in the art are either oil-based or water-based. Oil-based preservatives fall into two main classes, namely (i) coal tar creosote and solutions of creosote with coal tar or petroleum oils and (ii) solutions of Preservative chemicals, such as pentachlorophenol dissolved in a suitable organic carrier.
One disadvantage of oil based preservatives is that they exude from the wood. Thereafter they may wash from the surface or evaporate. In order to compensate for the loss of the oil-based preservative, high initial retentions are required. In tropical and high rainfall areas, the use of oil-based preservatives has been found to be uneconomical. Another disadvantage of certain oil-based preservatives is that they are regarded as skin irritants and can cause burns. Furthermore, oil-based preservatives such as creosote cannot be painted and do not have attractive appearances. These preservatives often have toxic side effects.
Water based preservatives are those containing chemical preservatives in the form of aqueous solutions. Such preservatives react within the wood to form compounds, the solubility of which may be increased by adjustment of pH. When chemical changes occur within the wood resulting in compounds with very low solubility, the compounds are designated as leach-resistant. Those which form soluble compounds are designated as leachable.
Leach-resistant water-based preservatives in commercial use include acid copper chromate solution (ACC), chromated copper arsenate solution (CCA) and ammoniacal copper arsenate solution (ACA). CCA solutions are commonly used. They form, in the wood, compounds which are toxic to both fungi and insects. Leachable water-based preservatives include chromated zinc chloride and fluoride-chromium-arsenate-phenol mixtures and boron compounds. The leachable water-based preservatives can only be used in treating timber to be used internally or where leaching conditions are not severe.
Use of water-based preservatives has many advantages: cleanliness, paintability of the treated wood, freedom from odor, and when correctly applied, longer protection of the wood.
The American Wood-Preserver's Association "Book of Standards" (1986) defines and describes on pages 1978 and 1979 well known techniques for applying wood preservative compositions. Among these are mentioned:
Brushing PA0 Butting PA0 Diffusion PA0 Dipping PA0 Double Pressure PA0 Dual PA0 Empty-Cell PA0 End Pressure PA0 Full-Cell PA0 Internal PA0 Non-Pressure PA0 Pressure PA0 Spray PA0 Surface PA0 Thermal PA0 Vacuum PA0 Brush: Application of one or more coats of liquids preservative to the surface of timber with a brush. PA0 Butt: Preservative treatment applied to the lower, or butt end of posts and poles; usually by the Thermal Process. PA0 Pressure: The impregnation of wood with a liquid by application of pressure above atmospheric or above any initial air pressure which may have been applied. PA0 Diffusion: A treatment in which green wood or water-soaked wood is immersed in an aqueous solution or has applied to it a paste or solid containing water-soluble chemicals, to permit the chemicals to diffuse into the water in the wood. PA0 Dip: Application of a liquid preservative to a wood by immersing the wood in the liquid for a short period of time. PA0 Dual: Treatment of wood to be used under severe conditions of exposure with two dissimilar synergistic preservatives in two separate treating cycles, e.g. treatment o marine piles and timbers for areas of extreme borer hazard. Usually, the first treatment is with a water-borne salt preservative; and the second with creosote or creosote-coal tar solution. PA0 Empty-Cell: A treatment in which air imprisoned in the wood is employed to force out part of the preservative when treating pressure is released and a final vacuum is applied. PA0 Internal: A treatment applied by injecting into a pole or timber, through holes bored for the purpose, sufficient preservative material to protect against deterioration from wood-destroying organisms. PA0 Non-Pressure: A process for treating wood which does not require the use of hydraulic pressure. PA0 Spray: Application of one or more coats of a liquid preservative to the surface of wood with a spraying device. PA0 Surface: Superficial application of a liquid preservative to wood by brushing, spraying, or dipping. PA0 Thermal: A process of impregnating wood by (a) submerging it in hot preservative or fluid for various lengths of time, and then (b) in preservative at a lower temperature, with resulting reduction of pressure within the wood and forcing of the preservative into the wood by atmospheric pressure. PA0 Vacuum: Application of treating liquids to wood in a closed vessel by evacuating or partially removing the air from the vessel and introducing the liquid without re-admitting air. PA0 Full-Cell: A treatment involving a preliminary vacuum followed by pressure impregnation such that the cell cavities in the treated portion of the wood remain partially or completely filled with preservative.
These methods achieve the desired result of applying compositions having wood preservation properties to wood. The descriptions of wood preservation methods described on these pages and throughout the Book of Standards are hereby incorporated by reference.
Essential features of the pressure method are that (1) the wood is surrounded by a preservative solution in a closed vessel; and (2) hydrostatic pressure is applied by mechanical means to force the solution into the wood fibers by replacing air or water already there, or going into any voids. It is conventional to evacuate the system to about 26 inches of mercury vacuum to remove air from cells within the wood. When a solution of CCA is used to impregnate wood, the CCA reacts inside the wood with reducing sugars found therein to form a mixture of insoluble salts.
U.S. Pat. No. 2,565,175 to Hager describes a method of preserving wood using specific types of preservatives in combination with specific methods and conditions of penetration and distribution of these preservatives within the wood. One specific type of preservative is CCA, to which ammonia is added to render the preservative solution alkaline. The ammonia addition prevents rapid fixation of the preservative in the wood. According to the method described by Hager, the preservative is introduced into the wood and the wood is kept in an undried condition for a period of time during which no fixation of the preservative occurs, and the preservative diffuses through the cell walls. Thereafter, the wood is dried.
U.S. Pat. No. 4,303,705 to Kelso, Jr. describes a Process for preserving wood against attack by living organisms, e.g., fungi and insects. The process may comprise one or two steps. In the two step process, there is a fungicidal step comprising introducing a copper solution into wood, and an insecticidal step comprising introducing a chromium and arsenic solution into wood.
One disadvantage of using CCA is that not all fixation of the preservative takes place in the wood. Sludging may occur in the working solution due to pickup of wood or wood extractives, corrosion, or impurities in the chemicals used to make up the solution. Sludging causes a deposit of solids on the surface of the wood. These deposits contain varying percentages of arsenic and thus are a matter of environmental concern. Recent treatment standards (AWPA 1982) have recognized this (see Hartford, W., "The Practical Chemistry of CCA in Service" , American Wood Preservers. Association Annual Meeting, Apr. 28, 29 and 30, 1986, pp. 1-16).
Lanthanide derivatives are used in glass, ceramic, paint, plastics, and rubber manufacture. Compositions comprising cerium compounds are known to have bactericidal effects, e.g. compositions comprising cerium nitrate and silver sulfadiazine (Boeck, et al., Burns vol. 11, no. 5 (1985) pp. 337-342; Monafo, 3rd International Congress on Pharmacological Treatment of Burns, Milan, Italy, May 12-15, 1980, Panmainerva Med., vol. 25, no. 3 (1983) pp. 151-154; Bowser, et al., J. Trauma, vol. 21, no. 7 (1981) pp. 558-563; Monafo, et al., Arch Surg. vol. 113, no. 4 (1978) pp. 397-401; Monafo, et al. Surgery (St. Louis) vol. 80, no. 4 (1976) pp. 465-473), and compositions containing electrically activated silver and cerium stearate (Colmano, et al., 23rd Annual Meeting of the Biophysical Society (New York), Atlanta, Ga., Feb. 26-28, 1979, Biophys. J. vol. 25, no. 2, part 2 (1979) P.2l7A). Cerium derivatives are also used as additives in plastics for food packaging.
U.S. Pat. No. 4,743,473 to Gradeff et al. incorporated herein by reference describes a method of preserving wood with lanthanide derivatives. The method involves the pressure treatment of wood using compositions comprising aqueous solutions of one or more lanthanide derivatives. It is believed that this treatment process produces wood containing leach-resistant lanthanide ions cross-linked with cellulose fibers in the wood. However, if U the wood is contacted with water for extended periods of time, it is possible that the lanthanide ions may eventually leach out.
U.S. Ser. No. 07/267,009 incorporated herein by reference (filed Nov. 4, 1988) now U.S. Pat. No. 4,883,689 to Gradeff also describes a method for preserving wood with lanthanide derivatives. In this process, wood is treated with organic solutions of one or more lanthanide derivatives. It is believed that this treatment process produces wood containing leach-resistant lanthanide complexes. Such complexes should be leach-resistant even when contacted with water for extended periods of time since the organo-soluble complexes deposited in the wood are water-insoluble. However, a potential drawback in this procedure is the use of organic solvents in the treatment process. Use and disposal of such chemicals may be undesirable on health and environmental grounds.
U.S. Pat. No. 4,090,000 to Hatcher discloses a two-step process to treat wood by first impregnating the wood with an aqueous solution of a polychlorophenate salt followed by the injection of an acid into the impregnated wood to precipitate the polychlorophenol in situ. However, Hatcher effectively teaches away from this process by noting that such a process requires a drying step, may require more elaborate facilities and most importantly, may prevent full permeation of the wood to be treated by formation of an outer perimeter plug of precipitated solid.
It is an object of the present invention to provide a new safe two-step method for treating wood or wood derived matrices with compositions comprising water-soluble lanthanide derivatives and then rendering said lanthanide derivatives leach-resistant by treatment of the wood with a second reactant solution comprising a base or a salt of an acid whereby water-insoluble lanthanide derivatives are produced within the wood. It is also an object of this invention to provide a means for impregnating wood with water-insoluble lanthanide derivatives without the use of potentially harmful organic solvents.
It is a further object of the invention to provide a variant of the aforementioned treatment process by reversing the order of treatment.
It is a further object of the present invention to promote flame retardation and to inhibit wood deterioration organisms such as exposure to wood-destroying organisms such as bacteria, insects and fungi to atmospheric conditions such as exposure to sunlight.
These and other objects are met by the present invention and are further described in the specification.